DNA damage causes mutation or cell death and is a contributing factor to carcinogenesis. Our laboratory has focused on DNA damage by electrophilic species generated endogenously by oxidative stress or inflammation. We are particularly interested in the lipid oxidation product, malondialdehyde (MDA), and the DNA oxidation products, base propenals. Previous work from our laboratory has provided a comprehensive inventory of adducts formed on reaction of MDA/base propenals with deoxynucleosides and DNA; demonstrated that the major adduct to deoxyguanosine (MldG) is mutagenic in bacterial and mammalian cells; revealed that MldG is repaired by nucleotide excision repair; and provided rigorous anal3^ical evidence that MldG is present endogenously in the human genome. In the most recent grant period, we have provided a detailed kinetic and structural analysis of the induction of mutations by MldG by Y-family translesion polymerases including a crystal structure of an MldG-containing template-primer bound in the active site of Dpo4; discovered that the major adduct of MDA/base propenals to deoxyadenosine, which is called OPdA, reacts with amino acids to form stable cross-links to DNA; and reported that MldG is present in the urine of healthy human subjects along with a single major metabolite, 6-oxo-MldG. These discoveries place us in an outstanding position to explore the chemical biology of DNA damage by endogenous products of oxidative stress and inflammation. Our specific aims for the next grant period are to (1) Determine the structural and functional consequences of the interaction of template-primers containing the endogenous adducts, MldG, OPdA, and heptanone-etheno-dG with Y-family DNA polymerases; (2) Explore the chemistry of DNA-DNA and DNA-protein cross-linking mediated by the endogenous adducts, OPdA and OPdC; and (3) Develop highly sensitive and specific analytical methods for quantification of MldG and 6-oxo-MldG that will allow simultaneous monitoring of both lesions in human populations and animal models.